docs: def check_impl(self):
          """
          returns a tuple of (is_change,description) which are then stored
          in self.changed and self.description
  
          The default implementation will get the data from the left and
          right sides by calling self.fn_data, then compare them via
          self.fn_differ. If they do differ, a message will be
          constructed using self.fn_pretty to create human-readable
          versions of the data that changed.
          """
  
          if self.fn_differ(self.get_ldata(), self.get_rdata()):
  
              left = self.pretty_ldata_desc()
              right = self.pretty_rdata_desc()
              msg = "%s changed: %s to %s" % (self.label, left, right)
  
              return True, msg
  
          else:
              return False, None